All references cited in this specification, and their references, are incorporated by reference herein where appropriate for teachings of additional or alternative details, features, and/or technical background.
Disclosed in the embodiments herein is an automated image diagnostics system that is activated by information extracted from an image-based control system indicative of degradation in image print quality.
In one embodiment, the present disclosure relates to electrophotographic printing. It finds particular application in conjunction with a method and system for controlling a printing device's tone reproduction curve. The invention helps to minimize the chance of degradation of image quality of final outputs by initiating an automated image quality diagnostics system via monitoring the control parameters in an image based control system. The invention will be described in reference to a xerographic print engine. However, the invention is also amenable to other electrophotographic processes, such as for example, ionographic print engines and like applications.
Electrophotographic copiers, printers and digital imaging systems typically record an electrostatic latent image on an imaging member. The latent image corresponds to the informational areas contained within a document being reproduced. In xerographic systems, a uniform charge is placed on a photoconductive member and portions of the photoconductive member are discharged by a scanning laser or other light source to create the latent image. In ionographic print engines the latent image is written to an insulating member by a beam of charge carriers, such as, for example, electrons. However it is created, the latent image is then developed by bringing a developer, including colorants, such as, for example, toner particles into contact with the latent image. The toner particles carry a charge and are attracted away from a toner supply and toward the latent image by an electrostatic field related to the latent image, thereby forming a toner image on the imaging member. The toner image is subsequently transferred to a physical media, such as a copy sheet. The copy sheet, having the toner image thereon, is then advanced to a fusing station for permanently affixing the toner image to the copy sheet.
The approach utilized for multi-color electrophotographic printing is substantially identical to the process described above. However, rather than forming a single latent image on the photoconductive surface in order to reproduce an original document, as in the case of black and white printing, multiple latent images corresponding to color separations are sequentially recorded on the photoconductive surface. Each single color electrostatic latent image is developed with toner of a color complimentary thereto and the process is repeated for differently colored images with the respective toner of complimentary color. Thereafter, each color toner image can be transferred to the copy sheet in superimposed registration with the other toner images, creating, for example, a multi-layered toner image on the copy sheet. This multi-layer toner image is permanently affixed to the copy sheet in substantially conventional manner to form a finished copy.
FIG. 4 is a simplified elevational view of essential elements of a color printer, showing a context of the present invention. Specifically, there is shown an “image-on-image” xerographic color printer, in which successive primary-color images are accumulated on a photoreceptor belt, and the accumulated superimposed images are in one step directly transferred to an output sheet as a full-color image.
Specifically, the FIG. 4 embodiment includes a belt photoreceptor 10, along which are disposed a series of stations, as is generally familiar in the art of xerography, one set for each primary color to be printed. For instance, to place a cyan color separation image on photoreceptor 10, there is used a charge corotron 12C, an imaging laser 14C, and a development unit 16C. For successive color separations, there is provided equivalent elements 12M, 14M, 16M (for magenta), 12Y, 14Y, 16Y (for yellow), and 12K, 14K, 16K (for black). The successive color separations are built up in a superimposed manner on the surface of photoreceptor 10, and then the combined full-color image is transferred at transfer station 20 to an output sheet. The output sheet is then run through a fuser 30, as is familiar in xerography.
Also shown in FIG. 4 is a set of what can be generally called “monitors,” such as 50 and 52, which can feed back to a control device 54. The monitors such as 50 and 52 are devices which can make measurements to images created on the photoreceptor 10 (such as monitor 50) or to images which were transferred to an output sheet (such as monitor 52). These monitors can be in the form of optical densitometers, calorimeters, electrostatic voltmeters, etc. There may be provided any number of monitors, and they may be placed anywhere in the printer as needed, not only in the locations illustrated. The information gathered therefrom is used by control device 54 in various ways to aid in the operation of the printer, whether in a real-time feedback loop, an offline calibration process, a registration system, etc.
For faster printing rates, requiring faster sheet feeding rates along paper paths, which can reach more than, for example, 100-200 pages per minute, registration systems and functions become much more difficult and expensive.
Systems that can perform image analysis on printed test samples can be used in a variety of ways to provide solutions and value to users of digital printers and copiers, for example as the analysis engine for automatic and/or remote diagnosis of print quality problems, or for monitoring image quality as part of a print quality assurance system. The image quality analysis may be used as part of a system for machine diagnostics. In such case, for example, images from a printer/copier may be scanned back by a stand-alone scanner or a scanner associated with the printer/copier, and fed into an image analysis module, which then quantifies different types of non-uniformities and use this as a basis for diagnosing machine problems. Non-uniformities include (a) amplitude modulated cluster dot halftone patterns; (b) frequency modulated halftone patterns (e.g., stochastic screens); (c) irregular two-dimensional variations from noise; (d) isolated (non-periodic) one-dimensional streaks; (e) periodic, one-dimensional bands; and (f) one- or two-dimensional periodic variations (Moire).
For example, non-uniformities in category (a) would be a result of the normal operation of the printer and not require any corrective action, while excessive non-uniformities in category (c) would be a diagnostic signal that the printer needs service. If separation between (a) and (c) is not made, the color variation caused by halftone screens could dominate the overall signal, and small but important variations caused by process noise may go undetected, resulting in ineffective diagnosis of printer/copier operation.
As far as the type of color variation is concerned, it may be variations purely in lightness (CIELab L*), or it could be variations that also include hue and chroma. Although the visual perception of such variations strongly depends on the type of color variation, the invention proposed here applies equally well to all of the above-identified types.
Amplitude modulated cluster dot halftone patterns usually have a relatively high spatial frequency (e.g., 141 lines per inch). These patterns are usually not very objectionable to a human observer. This is partly because the frequency is so high that they are not easily visible, and partly because of their regular, periodic nature. On the other hand, frequency modulated halftone patterns at the same level will be visible and highly objectionable by a human observer unless these are of high spatial frequency, because their irregular nature makes the print appear “noisy”. Irregular two-dimensional variations caused by various sources of noise in the printing process can form graininess or mottle in the image. For example, in an electrophotographic system, graininess is usually found in and caused by the development subsystem, while mottle is caused by an incomplete transfer of toner to substrate. Isolated (non-periodic) one-dimensional streaks, for example, can be caused by the signature of a misdirected jet for ink-jet printing. Periodic, one-dimensional bands, for example, can be caused by motion-quality problems with paper-advance mechanisms. One- or two-dimensional periodic variations known as Moire, can be caused by the interference of higher frequency periodic variations. This can be highly notable, for example, in three or four color printing when the different screens beat against each other. These are all types of non-uniformity which are addressed by the proposed technique.
It is well known that customer satisfaction can be improved and maintenance costs reduced if problems with copiers and printers can be fixed before they become serious enough to warrant a service call by the customer. While current technology exists to enable printers and copiers to call for service automatically when sensors detect certain operating parameters outside of permissible ranges, there is not a very comprehensive manner of detecting incipient system failure or automatically diagnosing when problems with image quality reach a level where human observers perceive a reduction in quality. This is caused not only by the large number of operating parameters that would need to be tracked, but also because these parameters are strongly coupled to one another. That is, a given parameter at a certain value may or may not be a problem depending on the values of other parameters. While existing systems may provide some level of image quality analysis, these systems may be found to be less than satisfactory as image quality determination is machine dependent and may be inconsistent with perceptions of image quality as judged by human users.